Filtration is a well known method to separate impurities from liquids or gases. It is generally used to remove solids, but may also be used to separate compounds of various sizes as in the use of molecular sieves. Systems are available that combine preferential absorption along with the physical separation of particles occurring in filtration. For example, it is well known that activated carbon may be used to extract a broad range of chemicals from liquids. Thus, when it is added to a filter media, particles of various sizes that are too large to pass through the filter media are separated and also certain materials that are absorbed by the carbon as a component of the total media are removed.
These materials are what might be called active particles in that they improve the separation through absorption or some other method. In many filtration processes, it is desirable to have these particles built into the filter medium. When used alone, most of these form density packed beds making it impossible to use them in performing continuous filtration of liquid streams. Therefore, it is desirable to have this type material in small quantities in a dispersed state for most effective use.
Aggregates used as the principal filtering substance must have certain physical properties. These are:
1. Good porosity.
2. Pore sizes sufficient to allow the desired material to pass through yet prevent the undesirable material from passing through.
3. They should not readily compact.
4. They should not form a sticky mass like clays when wet.
5. They should be dimentionally stable at the temperature and pressure range where the filtration occurs.
It would be highly desirable to have an aggregate with all of the above properties. It would be further advantageous to also have a material with a small percentage of active particulates uniformly dispersed throughout the filter medium, such as activated carbon or water-swellable polymers providing additional filtering properties that are highly useful in many filtration processes.